Add support for websockets (#3)

Basically a copy/paste of whats in warp.

Example usage:

```rust
use tower_web::{prelude::*, ws::{ws, WebSocket}};

let app = route("/ws", ws(handle_socket));

async fn handle_socket(mut socket: WebSocket) {
    while let Some(msg) = socket.recv().await {
        let msg = msg.unwrap();
        socket.send(msg).await.unwrap();
    }
}
```

src/lib.rs

@@ -610,6 +610,10 @@ pub mod response;
 pub mod routing;
 pub mod service;
 
+#[cfg(feature = "ws")]
+#[cfg_attr(docsrs, doc(cfg(feature = "ws")))]
+pub mod ws;
+
 #[cfg(test)]
 mod tests;